{-# LANGUAGE TupleSections #-}
{-# LANGUAGE ExplicitForAll #-}

-- | After the document is parsed, before getting executed the AST is
--   transformed into a similar, simpler AST. This module is responsible for
--   this transformation.
module Language.GraphQL.AST.Transform
    ( document
    ) where

import Control.Arrow (first)
import Control.Monad (foldM, unless)
import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Reader (ReaderT, ask, runReaderT)
import Control.Monad.Trans.State (StateT, evalStateT, gets, modify)
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HashMap
import qualified Data.List.NonEmpty as NonEmpty
import Data.Sequence (Seq, (<|), (><))
import qualified Language.GraphQL.AST as Full
import qualified Language.GraphQL.AST.Core as Core
import qualified Language.GraphQL.Schema as Schema

-- | Associates a fragment name with a list of 'Core.Field's.
data Replacement = Replacement
    { fragments :: HashMap Core.Name (Seq Core.Selection)
    , fragmentDefinitions :: HashMap Full.Name Full.FragmentDefinition
    }

type TransformT a = StateT Replacement (ReaderT Schema.Subs Maybe) a

-- | Rewrites the original syntax tree into an intermediate representation used
-- for query execution.
document :: Schema.Subs -> Full.Document -> Maybe Core.Document
document subs document' =
    flip runReaderT subs
        $ evalStateT (collectFragments >> operations operationDefinitions)
        $ Replacement HashMap.empty fragmentTable
  where
    (fragmentTable, operationDefinitions) = foldr defragment mempty document'
    defragment (Full.DefinitionOperation definition) acc =
        (definition :) <$> acc
    defragment (Full.DefinitionFragment definition) acc =
        let (Full.FragmentDefinition name _ _ _) = definition
         in first (HashMap.insert name definition) acc

-- * Operation

-- TODO: Replace Maybe by MonadThrow CustomError
operations :: [Full.OperationDefinition] -> TransformT Core.Document
operations operations' = do
    coreOperations <- traverse operation operations'
    lift . lift $ NonEmpty.nonEmpty coreOperations

operation :: Full.OperationDefinition -> TransformT Core.Operation
operation (Full.OperationSelectionSet sels) =
    operation $ Full.OperationDefinition Full.Query mempty mempty mempty sels
-- TODO: Validate Variable definitions with substituter
operation (Full.OperationDefinition Full.Query name _vars _dirs sels) =
    Core.Query name <$> appendSelection sels
operation (Full.OperationDefinition Full.Mutation name _vars _dirs sels) =
    Core.Mutation name <$> appendSelection sels

selection ::
    Full.Selection ->
    TransformT (Either (Seq Core.Selection) Core.Selection)
selection (Full.SelectionField fld) = Right . Core.SelectionField <$> field fld
selection (Full.SelectionFragmentSpread (Full.FragmentSpread name _)) = do
    fragments' <- gets fragments
    Left <$> maybe lookupDefinition liftJust (HashMap.lookup name fragments')
  where
    lookupDefinition :: TransformT (Seq Core.Selection)
    lookupDefinition = do
        fragmentDefinitions' <- gets fragmentDefinitions
        found <- lift . lift $ HashMap.lookup name fragmentDefinitions'
        fragmentDefinition found
selection (Full.SelectionInlineFragment fragment)
    | (Full.InlineFragment (Just typeCondition) _ selectionSet) <- fragment
        = Right
        . Core.SelectionFragment
        . Core.Fragment typeCondition
        <$> appendSelection selectionSet
    | (Full.InlineFragment Nothing _ selectionSet) <- fragment
        = Left <$> appendSelection selectionSet

-- * Fragment replacement

-- | Extract fragment definitions into a single 'HashMap'.
collectFragments :: TransformT ()
collectFragments = do
    fragDefs <- gets fragmentDefinitions
    let nextValue = head $ HashMap.elems fragDefs
    unless (HashMap.null fragDefs) $ do
        _ <- fragmentDefinition nextValue
        collectFragments

fragmentDefinition ::
    Full.FragmentDefinition ->
    TransformT (Seq Core.Selection)
fragmentDefinition (Full.FragmentDefinition name _tc _dirs selections) = do
    modify deleteFragmentDefinition
    newValue <- appendSelection selections
    modify $ insertFragment newValue
    liftJust newValue
  where
    deleteFragmentDefinition (Replacement fragments' fragmentDefinitions') =
        Replacement fragments' $ HashMap.delete name fragmentDefinitions'
    insertFragment newValue (Replacement fragments' fragmentDefinitions') =
        let newFragments = HashMap.insert name newValue fragments'
         in Replacement newFragments fragmentDefinitions'

field :: Full.Field -> TransformT Core.Field
field (Full.Field a n args _dirs sels) = do
    arguments <- traverse argument args
    selection' <- appendSelection sels
    return $ Core.Field a n arguments selection'

argument :: Full.Argument -> TransformT Core.Argument
argument (Full.Argument n v) = Core.Argument n <$> value v

value :: Full.Value -> TransformT Core.Value
value (Full.Variable n) = do
    substitute' <- lift ask
    lift . lift $ substitute' n
value (Full.Int i) = pure $ Core.Int i
value (Full.Float f) = pure $ Core.Float f
value (Full.String x) = pure $ Core.String x
value (Full.Boolean b) = pure $ Core.Boolean b
value Full.Null = pure   Core.Null
value (Full.Enum e) = pure $ Core.Enum e
value (Full.List l) =
    Core.List <$> traverse value l
value (Full.Object o) =
    Core.Object . HashMap.fromList <$> traverse objectField o

objectField :: Full.ObjectField -> TransformT (Core.Name, Core.Value)
objectField (Full.ObjectField n v) = (n,) <$> value v

appendSelection ::
    Traversable t =>
    t Full.Selection ->
    TransformT (Seq Core.Selection)
appendSelection = foldM go mempty
  where
    go acc sel = append acc <$> selection sel
    append acc (Left list) = list >< acc
    append acc (Right one) = one <| acc

liftJust :: forall a. a -> TransformT a
liftJust = lift . lift . Just